1. Field of the Invention
This invention relates to circuitry for processing radio frequency signals and, more specifically, to a mixer circuit.
2. Description of the Prior Art
Mixer circuits ("mixers") are well-known in the electronics field and especially in the RF (radio frequency) field. A mixer converts a high frequency (RF) signal to a lower frequency, or as it is usually called, intermediate frequency (IF) signal which is more easily processed, in for instance by a radio receiver, for purpose of amplification. What is commonly called a mixer is more accurately a converter or frequency converter. (This disclosure uses the more common term mixer to avoid confusion with other types of converters.) Mixers perform similar functions in both transmitting and receiving circuits. Many types of mixers are known. An ideal mixer converts any input RF signal to another lower chosen frequency with no distortion and has a noise figure of 0 dB. Such a mixer, of course, does not exist. However, it is desirable to minimize noise generated by the mixer circuit. Typically a mixer receives a local oscillator (LO) signal of a fixed frequency generated by a local oscillator in the radio and also receives the RF signal, and converts the RF signal to a substantially lower IF frequency. The IF frequency is typically the difference between the RF signal frequency and local oscillator frequency. A conventional mixer, when used to demodulate an IF signal, demodulates any RF frequency either above and below the LO frequency down to the IF frequency. In other words, if the RF signal is at 260 MHz and the LO signal is at 250 MHz, a conventional mixer down converts the RF signal to 10 MHz, which is the difference between the RF frequency and LO frequency. This is a demodulation of the upper sideband and would be a desirable result. However, if there is also an RF signal present at 240 MHz, the conventional mixer will also down convert this signal to 10 MHz. This demodulation of the lower sideband is not desired in a single sideband radio system.
An image reject mixer is an application for a mixer. It demodulates the upper sideband of the RF signal down to an IF frequency, while rejecting the lower sideband, which are the RF signals below the local oscillator frequency. It can also do the reverse: demodulate lower sideband while rejecting the upper sideband, when configured properly. An image reject mixer is described in "Image Reject and Image Canceling Mixers" Pandula, RF Design, April, 1995, pp. 60 and following. This shows an image reject mixer with two mixer circuits with common RF input terminals and LO inputs that are in quadrature (90.degree. apart in terms of phase). This is also known as a quadrature modulator or demodulator, and shows use of mixer circuits.
In general, it is well-known that any type of mixer circuit should have low noise, high dynamic range (both of the input and output signals), and an easily attained conversion gain and an easily supplied current level. It is known that for image reject mixers, noise is a particular problem because the two separate mixer circuits having common inputs tend to generate noise. Available mixer circuits do not provide adequate low noise performance for such applications.